1. Field of the Invention
The present invention relates to a combustion heater, and more specifically, to the combustion heater which prevents incomplete combustion for improving emissions in the exhaust gas.
2. Description of the Prior Art
As is known in the art, the combustion heater is arranged to burn up a portion of an engine fuel with a burner for directly heating air in a vehicular compartment or heating water to be conducted to a radiator in the vehicular compartment. The combustion heater is used as an auxiliary vehicle heater before a temperature of the engine cooling water increases to a given high value.
In the burner of the combustion heater, the supplied fuel is vaporized and mixed with combustion air, and this mixture is ignited and burned using a heater or a glow plug.
FIG. 23 shows one example of the conventional combustion heaters. In the figure, a housing 1 has a double-wall structure to define a fluid passage 13 for water to be conducted to a radiator (not shown). A burner 2 is arranged in the housing 1. The burner 2 has a burner tube which is provided with a vaporization plate 4 at one axial end thereof. The vaporization plate 4 is prepared by forming ceramic fibers into a plate shape. An open end of a fuel feed pipe 73 is in abutment with the vaporization plate 4. Fuel is fed to the fuel feed pipe 73 from a pump 72 in a fuel tank 71 via a fuel cutoff valve 76. The fuel then reaches the vaporization plate 4 where the fuel is dispersed over the entirety of the vaporization plate 4 due to the capillary action. The vaporization plate 4 is heated by a glow plug 9 provided adjacent thereto. Accordingly, the fuel is vaporized from the vaporization plate 4 and then mixed with combustion air introduced into the burner tube via air introducing holes 211 so as to be ignited to burn. The combustion air is supplied from an air pump 82 provided at an air feed pipe 81 extending to an air inlet of the housing 1.
An electronic control unit (ECU) 6 is provided for controlling an operation of the pump 72, an opening/closing operation of the fuel cutoff valve 76, and energization of the glow plug 9. The ECU 6 further controls an operation of the air pump 82.
The combustion heater using such a vaporization plate is disclosed in, such as, Japanese First (unexamined) Patent Publication No. 1-262214, Japanese First (unexamined) Utility Model Publication No. 3-71251, Japanese First (unexamined) Patent Publication No. 4-73503, Japanese First (unexamined) Patent Publication No. 4-214105 equivalent to U.S. Pat. No. 5,088,918, or U.S. Pat. No. 4,538,985.
For further prior art information, Japanese First (unexamined) Patent Publication No. 60-29505 shows a catalytic heater in which fuel is atomized using an ultrasonic vibrator.
In the conventional combustion heaters, however, a portion of the supplied fuel does not reach a combustion temperature at the time of ignition since the burner is cold at that time. This increases HC emissions in the form of unburned gas. Similarly, a problem is also raised at the time of extinction that, since the fuel supply is suddenly stopped, the fuel remaining in the vaporization plate becomes unburned gas to increase HC emissions.
In Japanese First (unexamined) Utility Model Publication No. 3-71251 as referred to above, the fuel supply is suddenly stopped at the time of extinction, while an air supply mount at the time of the fuel supply stop is held for a given short time period thereafter. FIGS. 24A and 24B show, respectively, variations in fuel supply amount and air supply amount at the time of extinction of the combustion heater. As seen from these figures, in response to a combustion stop signal, the ECU 6 immediately stops the fuel supply, while maintaining the air supply amount corresponding to the combustion power of 100% for a given time period T0 for purging the exhaust gas including HC and for cooling the burner.
As described above, in the combustion heater using the vaporization plate 4, the fuel supplied to the vaporization plate 4 is first dispersed over the entirety thereof due to the capillary action and then, after being held at the vaporization plate 4 for a while, vaporized from the vaporization plate 4. Accordingly, when the fuel supply is stopped, the fuel still remains largely at the vaporization plate 4. Thus, if a large amount of the air supply is held in this condition to cause deviation of an air-fuel ratio of the supplied air and the vaporizing remaining fuel from a proper air-fuel ratio, HC emissions are increased as shown in FIG. 24C.
Further, in the foregoing combustion heaters, as described above, it takes some time for the fuel supplied to the vaporization plate 4 to be vaporized from the vaporization plate 4. On the other hand, the combustion air is directly introduced into the burner tube via the air introducing holes 211. Accordingly, when the fuel and the air are supplied synchronously, the air is oversupplied at the initial stage of the combustion to deteriorate the exhaust gas emissions. This applies not only to the initial stage of the combustion but also when increasing the fuel supply amount for enhancing the heating power. Further, it also takes some time for the vaporizing fuel to be decreased after stopping or reducing the fuel supply at the time of extinction or at the time of lowering the heating power. This causes the fuel to be oversupplied to thus deteriorate the exhaust gas emissions.